Sediment Dispersion and Mound Development and Site Geometry During Placement

Sediment Dispersion and

Mound Development and

Site Geometry During

Placement

The physical behavior of a dredged material discharge depends on the

type of dredging and disposal operation used, nature of the material (physi-

cal characteristics), and hydrodynamics of the disposal site. For capping

operations, it is essential to determine beforehand the nature of the dis-

charge for both contaminated and capping material. The degree of disper-

sion and associated water column contaminant release dictates whether a

given discharge is acceptable from the standpoint of water column im-

pacts. The geometry of the subaqueous deposit or mound dictates the re-

quired area to be capped and cap configuration.

Sediment Dispersion During Placement

A knowledge of the short-term physical fate of both the contaminated

material and capping material is necessary to determine the acceptability

of the equipment and placement operation under consideration. Short-

term fate is defined as the behavior exhibited by the material during and

immediately following discharge. The dispersion of material released into

the water column and the deposition of the material on the bottom are also

of interest. These processes occur over a time period of a few minutes to

several hours for a single release from a barge or hopper dredge.

In addition to physical dispersion of suspended material, an evaluation

of water column mixing of released contaminants or suspended dredged

material is necessary whenever potential water column contaminant ef-

fects are of concern. Such an evaluation may involve comparison of pre-

dicted water column contaminant concentrations with water quality

criteria (or standards) or predicted suspended dredged material concentra-

tions with bioassay test results. Water column effects measured in the

field on actual projects may be valuable in quantifying water quality ef-

fects. For capping operations, such evaluations are normally required for

the contaminated material to determine if water column control measures

Chapter 6 Sediment Dispersion and Mound Development and Site Geometry During Placement